energy: fix shell command in docs

src/energy/examples/generic-battery-discharge-example.cc

@@ -44,8 +44,10 @@ using namespace ns3;
  *
  * Plot files are produced as a result of this example.
  * Graphs can be obtained from the plot using:
+ * \code{.sh}
+   $> gnuplot <plotname>.plt
+   \endcode
  *
- * $> gnuplot <plotname>.plt
  */
 
 Gnuplot battDischPlot1 = Gnuplot("BattDisch1.eps");